Opp = Opp Power is largely considered unplayable. Even cards with a bonus that makes up for that ability, if it should be shut down, are considered shaky. When was the last time you've seen someone use poor Angie? Furthermore, a lot of power reducers that are equivalent to 7 power feel outdated now -- even ones that were once considered great, like Tula! Attack manipulation, SoA, Prot: Power, and 8 powered cards, all destroy those cards with ease.

Opp = Opp Power +1 allows cards of a similar nature to exist. They would have a small advantage against 8 powered cards and Prot: Power. However, they would still be at a disadvantage against attack manip cards, power manip cards, and SoA. As long as they dealt reasonable damage and were not found in certain clans, that would be enough to keep them from becoming overpowered. For example, a card like that in a SoB clan should have somewhat low damage for its star count.